Add options for the Julia binary and project path

This makes using JuliaPkg optional.

Author: JamesWrigley

.github/workflows/docs.yml

@@ -23,7 +23,6 @@ jobs:
       - run: |
           julia --project=docs -e '
             using Pkg
-            Pkg.develop(PackageSpec(path=pwd()))
             Pkg.instantiate()'
       - run: |
           julia --project=docs -e '

docs/Project.toml

@@ -4,3 +4,6 @@ PythonCall = "6099a3de-0909-46bc-b1f4-468b9a2dfc0d"
 
 [compat]
 Documenter = "1"
+
+[sources]
+PythonCall = {path = ".."}

docs/src/juliacall.md

@@ -73,7 +73,8 @@ What to read next:
 
 ## [Managing Julia dependencies](@id julia-deps)
 
-JuliaCall manages its Julia dependencies using [JuliaPkg](https://github.com/JuliaPy/PyJuliaPkg).
+By default JuliaCall manages its Julia dependencies using
+[JuliaPkg](https://github.com/JuliaPy/PyJuliaPkg).
 
 It will automatically download a suitable version of Julia if required.
 
@@ -100,6 +101,15 @@ Alternatively you can use `add`, `rm`, etc. from JuliaPkg to edit this file.
 
 See [JuliaPkg](https://github.com/JuliaPy/PyJuliaPkg) for more details.
 
+### Using existing environments
+
+It's possible to override the defaults and disable JuliaPkg entirely by setting
+the `PYTHON_JULIACALL_EXEPATH` and `PYTHON_JULIACALL_PROJECT` options. This is
+particularly useful when using shared environments on HPC systems that may be
+readonly. If `PYTHON_JULIACALL_PROJECT` is specified the project *must* already
+have PythonCall.jl already installed. If only one of these options is specified
+then JuliaPkg will still be loaded to get a default value for the other one.
+
 ## [Configuration](@id julia-config)
 
 Some features of the Julia process, such as the optimization level or number of threads, may
@@ -125,6 +135,8 @@ be configured in two ways:
 | `-X juliacall-warn-overwrite=<yes\|no>` | `PYTHON_JULIACALL_WARN_OVERWRITE=<yes\|no>` | Enable or disable method overwrite warnings. |
 | `-X juliacall-autoload-ipython-extension=<yes\|no>` | `PYTHON_JULIACALL_AUTOLOAD_IPYTHON_EXTENSION=<yes\|no>` | Enable or disable IPython extension autoloading. |
 | `-X juliacall-heap-size-hint=<N>` | `PYTHON_JULIACALL_HEAP_SIZE_HINT=<N>` | Hint for initial heap size in bytes. |
+| `-X juliacall-exepath=<file>` | `PYTHON_JULIACALL_EXEPATH=<file>` | Path to Julia binary to use (overrides JuliaPkg). |
+| `-X juliacall-project=<dir>` | `PYTHON_JULIACALL_PROJECT=<dir>` | Path to the Julia project to use (overrides JuliaPkg). |
 
 ## [Multi-threading](@id py-multi-threading)
 

docs/src/releasenotes.md

@@ -3,6 +3,7 @@
 ## Unreleased
 * Bug fixes.
 * Internal: switch from Requires.jl to package extensions.
+* Added support for specifying the Julia binary and project to override JuliaPkg.
 
 ## 0.9.27 (2025-08-19)
 * Internal: Use heap-allocated types (PyType_FromSpec) to improve ABI compatibility.

pysrc/juliacall/__init__.py

@@ -147,18 +147,29 @@ def args_from_config(config):
     CONFIG['opt_handle_signals'] = choice('handle_signals', ['yes', 'no'])[0]
     CONFIG['opt_startup_file'] = choice('startup_file', ['yes', 'no'])[0]
     CONFIG['opt_heap_size_hint'] = option('heap_size_hint')[0]
+    CONFIG['opt_project'] = path_option('project', check_exists=True)[0]
+    CONFIG['exepath'] = path_option('exepath', check_exists=True)[0]
 
     # Stop if we already initialised
     if CONFIG['inited']:
         return
 
-    # we don't import this at the top level because it is not required when juliacall is
-    # loaded by PythonCall and won't be available
-    import juliapkg
-
-    # Find the Julia executable and project
-    CONFIG['exepath'] = exepath = juliapkg.executable()
-    CONFIG['project'] = project = juliapkg.project()
+    no_exepath = CONFIG['exepath'] is None
+    no_project = CONFIG['opt_project'] is None
+    if no_exepath or no_project:
+        # we don't import this at the top level because it is not required when
+        # juliacall is loaded by PythonCall and won't be available, or if both
+        # `exepath` and `project` are set by the user.
+        import juliapkg
+
+        # Find the Julia executable and project
+        if no_exepath:
+            CONFIG['exepath'] = juliapkg.executable()
+        if no_project:
+            CONFIG['opt_project'] = juliapkg.project()
+
+    exepath = CONFIG['exepath']
+    project = CONFIG['opt_project']
 
     # Find the Julia library
     cmd = [exepath, '--project='+project, '--startup-file=no', '-O0', '--compile=min',